The genus Oenothera (evening primrose) displays a unique combination of non-Mendelian genetic features, like permanent translocation heterozygosity and biparental transmission of plastids. They allow the exchange of plastids between species, which often results in plastome-genome incompatibility (PGI) – a hybridization barrier according to the Dobzhansky-Muller model. In turn, PGI provides molecular access to speciation forces acting on photosynthesis. Furthermore, Oenothera exhibits over 20 transitions between sexual and functionally asexual reproduction, caused by chromosomal translocations and meiotic ring formation. These features make Oenothera an ideal model for examining hypotheses regarding the evolutionary advantages of sex, as well as of speciation.

To address these questions we established Oenothera as a molecular model over the past decade. We could indentify first plastid candidate loci for PGI, involved in chloroplast gene regulation. Moreover, the first genetic map of sexual Oenothera species has been generated assigning more than 1,700 dominant and co-dominant molecular markers to seven coupling groups. Remarkably, homologous recombination was almost completely undetectable in F2 segregants and restricted to the telomeric regions of the chromosomes. We could further demonstrate that an astonishing compartmentation into two chromatin fractions exists in the evening primrose nuclear genome. Remarkably, this genomic architecture promotes the concentration of translocation breakpoints in the nuclear territory between the two chromatin domains, thus facilitating exchanges of euchromatic end segments as the mechanism for the formation of meiotic rings. Reshuffling of chromosomal end segments, each possessing a cross-over hotspot is the first model that can fully explain the Oenothera translocation system and its astonishing genetic behaviour.